The present invention relates generally to both discrete devices and integrated circuits, and more particularly to a merged semiconductor device having a Schottky diode and a method for forming the device. The semiconductor device uses the layout area more efficiently, and thus may be smaller, than prior-art merged semiconductor devices that include Schottky diodes.
MOS-gated semiconductor devices, such as transistors, IGBTs, and MCTs, are used in many of today""s electronic applications. For example, FIGS. 1A and 1B are respectively a cross-sectional and a schematic symbol of a vertical DMOS transistor 10. The transistor 10 includes a drain contact 12, which is disposed on a side of an N+ semiconductor substrate 14. An Nxe2x88x92 epitaxial layer 16 is disposed on the other side of the substrate 14 such that the layer 16 acts as the drain and the substrate 14 acts as the drain contact region of the transistor 10. P-body regions 18 are disposed in the layer 16, and N+ source regions 20 are disposed in the body regions 18. A gate 21 is disposed over the body regions 18 and is isolated therefrom by a gate insulator 22. Source/body contacts 23 are disposed on the layer 16 in contact with both the body regions 18 and the source regions 20. These contacts allow the body and source regions 18 and 20 to be biased to the same voltage as is desired in many applications. As a consequence, however, the body regions 18 form the anode and the layer 16 forms the cathode of a PN diode 24. Fortunately, as discussed below, this xe2x80x9cbuilt-inxe2x80x9d diode serves to protect the transistor 10 from damage if the source voltage exceeds the drain voltage. Furthermore, the gate 21, contacts 23, and the underlying body and source regions 18 and 20 may be cellular structures such as squares, hexagons, or octagons, may be a meshed structure, may be interdigitated or striped, or may be other well-known geometries.
During a typical period of operation, the voltage on drain contact 12 is more positive than the source voltage on the source/body contacts 23, and the gate voltage on the gate 21 is greater than or equal to one threshold voltage above the source voltage. These conditions cause a channel region to form at the tops of the body regions 18 between the respective source regions 20 and the drain layer 16 so that a drain-to-source current flows from the drain contact 12, through the substrate 14, the layer 16, the channel regions, and the source regions 20, to contacts 23.
Conversely, during a transient period, the source voltage of the transistor 10 may become greater than the drain voltage. With the diode 24, however, if the source voltage would otherwise exceed the drain voltage by more than the forward voltage of the diode 24 (typically 0.7 V for a silicon diode), then the diode 24 conducts a current from the source contacts 23, through the body regions 18, layer 16, and substrate 14, to the drain contact 12. Thus, the diode 24 limits the source-to-drain voltage to approximately one diode drop.
Unfortunately, the conduction of a current by the diode 24 during such a transient period may adversely affect the subsequent operation of the transistor 10. More specifically, when the diode 24 conducts a current to limit the source-to-drain voltage of the transistor 10, minority carriers, here xe2x80x9cholesxe2x80x9d, are injected from the P body regions 18 into the Nxe2x88x92 drain layer 16. In some instances, the minority carriers in the drain layer 16 will continue to support a flow of current through the diode 24 even after the source voltage becomes less than the drain voltage. In some applications, this continuing current flow may hinder or prevent the desired operation of the transistor 10.
To prevent the diode 24 from conducting current when the source voltage exceeds the drain voltage, a Schottky diode having a lower forward voltage can be added in parallel to the diode 24. As discussed below, because of its lower forward voltage, the Schottky diode will both protect the transistor 10 and prevent the diode 24 from conducting a current. A Schottky diode also does not introduce minority carriers into the drain region 16, preventing the problems that occur when minority carriers are introduced by a PN junction.
FIGS. 2A-2B are respectively a cross-section and a schematic symbol of a vertical DMOS transistor 30, which is similar to the transistor 10 of FIGS. 1A-1B except that it includes a built-in Schottky diode 32. The Schottky diode 32 is shown in the exploded section of FIG. 2A and in FIG. 2B. For clarity, like reference numerals are used FIGS. 2A-2B for elements common to FIGS. 1A-1B.
Referring to FIG. 2A, the transistor 30 has outer source/body regions 34, which include N+ source regions 36 and P body regions 38, and also has inner source/body regions 40, which include N+ source regions 42 and a P body regions 43. A gate 44 is disposed over the P body regions 38 and 43 and is insulated therefrom by a gate insulator 45. Outer source/body contacts 46 contact the source regions 36 and the body regions 38, and a source/body/Schottky contact 48 contacts the source regions 42 and the body regions 43 as well as the drain layer 16. During operation, the contacts 46 and 48 are electrically coupled together. The contact 48 includes a Schottky contact 50, which contacts the drain layer 16. Thus, the contact 50 forms the anode and the drain layer 16 forms the cathode of the Schottky diode 32. The contact 50 also contacts the source and body regions 42 and 43, and thus acts as an ohmic contact thereto. The contact 48 also includes a layer 51 of metal disposed on the Schottky contact 50. A built-in PN junction diode 52, which is similar to the diode 24 of FIGS. 1A-1B, is formed by parallel diodes 53 and 54. The P body regions 38 and 43 form the anodes of the diodes 53 and 54, respectively, and the Nxe2x88x92 drain layer 16 forms a common cathode for the diodes 53 and 54. As discussed below, so that the diode 52 does not turn on during a transient period, the Schottky diode 32 is constructed to have a lower forward voltage than the PN junction diode 52. For example, using conventional techniques, the Schottky diode 32 can be constructed to have a forward voltage of 0.3-0.5V, which is less than the 0.7V forward voltage of the diode 52. The device shown in cross-section in FIG. 2A may have any of the surface geometries that the device of FIG. 1A has.
During a typical period of operation, the transistor 30 operates in a manner similar to that described above for the transistor 10.
During a transient period, if the source voltage would otherwise exceed the drain voltage by more than the forward voltage of the Schottky diode 32, then the diode 32 conducts a current from the metal 51, through the Schottky contact 50, layer 16, and substrate 14, to the drain contact 12. Thus, the diode 32 protects the transistor 30 by limiting the source-to-drain voltage to the diode 32 forward voltage. Furthermore, because the forward voltage of the Schottky diode 32 is less than that of the diode 52, the diode 52 does not turn on, and thus does not cause minority carriers to be injected into the layers 14 and 16.
Unfortunately, the Schottky diode 32 occupies a relatively large layout area, and thus significantly increases the layout area of the transistor 30 as compared to the transistor 10 of FIGS. 1A-1B. Furthermore, the reverse breakdown voltage of the Schottky diode 32xe2x80x94the maximum value by which the voltage on the drain layer 16 can exceed the voltage on the contact 50 without causing the diode 32 to break downxe2x80x94is often relatively low. Thus, Schottky diode 32 may lower the maximum drain-to-source voltage of the transistor 30 below that of the transistor 10. Additionally, the processes available to manufacture the transistor 30 are often relatively complex and require relatively large numbers of mask and other processing steps.
In one aspect of the invention, a semiconductor device includes a semiconductor substrate having a first conductivity and a semiconductor layer disposed on the substrate and also having the first conductivity. A recess is disposed in the layer and has a sidewall and a bottom. A gate insulator is disposed on the layer and extends to the sidewall of the recess, and a gate is disposed on the gate insulator. A body region is disposed in the semiconductor layer beneath the gate, and has a second conductivity and is contiguous with the sidewall of the recess. A source region is disposed in the body region, has the first conductivity, and is contiguous with the sidewall. A Schottky contact is disposed on the bottom of the recess, and a source metallization is disposed on the Schottky contact and on the sidewall of the recess.
Such a semiconductor device requires no additional area for the built-in Schottky diode. Furthermore, in another aspect of the invention, the built-in Schottky diode has an increased reverse-breakdown voltage. Additionally, such a device can be manufactured using a simplified manufacturing process.